The natural sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Some types of conductive hearing loss occur when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea, which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from severe to profound sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers by way of an array of electrodes implanted within the cochlea. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
Typically, cochlear implant systems utilize either a Continuous Interleaved Stimulation (“CIS”) strategy or a Simultaneous Analog Stimulation (“SAS”) strategy to represent an audio signal to a patient. In SAS, all of the electrodes in an electrode array are stimulated simultaneously during a stimulation frame. Because of this, SAS requires a relatively high amount of power. CIS requires less energy and hence is more power efficient than SAS, but relies on a bi-phasic pulse in which a second pulse is provided right after a first pulse during a stimulation frame to retrieve or balance the charge delivered to the electrode by the first pulse. The second pulse, which does not contribute to loudness growth of the first pulse, results in wasted energy.